Fair Value Measurements - Financial Instruments Measured at Fair Value on a Recurring and Non-recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	$ 494,901	$ 1,434,801
U.S. Treasury [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	1,996	2,004
U.S. Government Sponsored Enterprises [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	9,998	130,340
SBA Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	7,431	8,156
U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	429,309	1,233,357
Privately Issued Residential Mortgage Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	2,418	2,947
Obligations Issued by States and Political Subdivisions [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	41,013	55,174
Other Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	2,189	2,253
Equity Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	547	570
Carrying Value [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	494,901
Financial Instruments Measured at Fair Value on a Non-recurring Basis
Impaired Loans	2,967
Carrying Value [Member] | U.S. Treasury [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	1,996
Carrying Value [Member] | U.S. Government Sponsored Enterprises [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	9,998
Carrying Value [Member] | SBA Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	7,431
Carrying Value [Member] | U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	429,309
Carrying Value [Member] | Privately Issued Residential Mortgage Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	2,418
Carrying Value [Member] | Obligations Issued by States and Political Subdivisions [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	41,013
Carrying Value [Member] | Other Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	2,189
Carrying Value [Member] | Equity Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	547
Fair Value Measurements, Level 1 Inputs [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	256
Fair Value Measurements, Level 1 Inputs [Member] | Equity Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	256
Fair Value Measurements, Level 2 Inputs [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	454,166
Fair Value Measurements, Level 2 Inputs [Member] | U.S. Treasury [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	1,996
Fair Value Measurements, Level 2 Inputs [Member] | U.S. Government Sponsored Enterprises [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	9,998
Fair Value Measurements, Level 2 Inputs [Member] | SBA Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	7,431
Fair Value Measurements, Level 2 Inputs [Member] | U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	429,309
Fair Value Measurements, Level 2 Inputs [Member] | Privately Issued Residential Mortgage Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	2,418
Fair Value Measurements, Level 2 Inputs [Member] | Obligations Issued by States and Political Subdivisions [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	825
Fair Value Measurements, Level 2 Inputs [Member] | Other Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	2,189
Fair Value Measurements, Level 3 Inputs [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	40,479
Financial Instruments Measured at Fair Value on a Non-recurring Basis
Impaired Loans	2,967
Fair Value Measurements, Level 3 Inputs [Member] | Obligations Issued by States and Political Subdivisions [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	40,188
Fair Value Measurements, Level 3 Inputs [Member] | Equity Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Securities AFS	$ 291